Polyurethane Gels are often used for pressure distributing purposes, for example in mattresses, anti-fatigue mats, saddles, pressure distributing pads, etc.
A gel is defined here in accordance with IUPAC terminology as a “non-fluid polymer network that is expanded throughout its whole volume by a fluid”, a so-called swelling agent or extender. The fluid (discontinuous phase, dispersing phase, dispersant or fluid phase) may be physically or chemically bound within the network (network phase, continuous phase) which can be physically or chemically crosslinked. In a polyurethane gel (PU-gel), the crosslinked polyurethane backbone constitutes the continuous phase. The intrinsic physical properties of the gel allow it to support the user with an increased contact surface. Since the material is able to move in any direction without irreversible flow and distribute the weight avoiding discomfort and pressure marks.
EP 57838 (Burgdörfer et al.), EP 511570 (Schäpel et al.), and U.S. Pat. No. 4,404,296 (Schäpel) disclose polyurethane gels developed for the purposes outlined above. In these gels the polyurethane forming polyol functions not only as a reactant but also as a liquid dispersing agent or “expanding fluid” and thus constitutes the fluid phase of the polyurethane gel that is expanding the polyurethane network.
EP 2 789 270 A1 (Losio et al.), WO 2013/076661 A1 (Mason), and US 2013/0000045 A1 (Losio) disclose padding products and support elements, including mattresses with an upper gel structure providing a resting surface with high user comfort due to favourable cushioning and adaptability properties.
In addition to cushioning and optimal pressure distribution perfectly provided by the above-mentioned polyurethane gels, a satisfactory heat management is desired as well since polyurethane gels might suggest a cooling or heating effect to the user. Moreover, polyurethane and other polymer gels per se do not allow for air circulation. After only a short time, heat flow from the body ceases, e.g. in a shoe, an orthopaedic device or in other more or less closed environments. The user might then feel uncomfortable and sweat. An attempt to solve this problem is disclosed in US 2003/0088019 A1 (Pause et al.), in which polyurethane gel materials comprise finely divided Phase Change Materials (PCM)
The introduction of materials which absorb and store large quantities of heat from the surroundings during a phase change into matrices that do not change the physical state in the same temperature range, leads to a climatizing or temperature buffering effect. Mechanical and other properties of the product should be determined by the matrix while the temperature control is driven by PCM properties. However, there may be drawbacks such as limited PCM uptake in the matrix, change or loss of important matrix properties and loss of PCM by exudation during the product lifespan. In the latter case encapsulation of the PCM may be necessary to prevent leakage from the matrix. The effects caused by phase change materials entrapped in matrices are currently used in a wide variety of applications—inter alia in functional textiles, orthopaedic and sports equipment such as skiing boots, cushioning and insoles, as well as bedding products and building materials, e.g. wall or floor panels.
In general, a phase transition from the solid to the liquid state occurs on reaching the melting temperature during a heating process. During this melting process, the PCM absorbs and stores considerable latent heat. At least 160 J/g are desirable for a pronounced effect in a polymer matrix. The temperature of the PCM remains virtually constant during the entire process. During a subsequent cooling process, the stored heat is released again from the PCM to the surroundings while the reverse phase transition from the liquid to the solid state, the crystallisation, takes place. Again the temperature of the PCM remains constant during this process and there is a climate and temperature controlling effect regarding the overall device or product which is equipped with the PCM.
The temperature range of the phase change is adjusted for each application as required, for example at room temperature, at body temperature, at a certain climate control temperature, desired cooling temperature, etc. In most cases, the melting point needed for an application is targeted with the help of PCM mixtures.
Although a broad range of PCM mixtures is commercially available and all these materials have been studied in detail, the properties of PCM mixtures in polymer matrices are still unforeseeable. Some phase transitions tend to be irreversible or not fully reversible over a sufficient number of cycles.
For most PCM there may be a gap between the melting temperature Tm and the crystallisation temperature Tc which is highly undesired. This phenomenon is referred to as supercooling or undercooling. It occurs when a liquid is cooled below its melting point without becoming solid. In the worst case, the supercooling effect can deteriorate the desired temperature compensation and regulation.
Hydrocarbons and fats are an important group of phase change materials. They are most commonly used because of their low cost and low toxicity.
Paraffins have proved to be especially useful for an easy targeting of phase change ranges. Paraffin mixtures meet any PCM temperature specification from about 0-300° C. with poor or no supercooling effect. This seems to be due to very homologous molecular properties, esp. with respect to mixtures
US 2003/0088019 A1 (Pause et al.) discloses to incorporate saturated hydrocarbons or fats as PCM in a shock absorbing or cushioning material made of a polyurethane gel. This improves comfort when using the gel material in items such as shoe soles, bicycle seats, chair cushions, mattresses, etc. The paraffines are embedded directly into the PU matrix without encapsulation since encapsulation seriously slows down and deteriorates the heat transfer. Moreover, encapsulation is expensive and the capsules can break down in the soft PU-gel from wear. According to the concept described in US 2003/0088019 A1 the non-polar PCM is thoroughly emulsified in the more polar polyurethane mass, preferably within a polar polyol component, and entrapped in very finely dispersed droplets within the matrix. Unfortunately, paraffins and fats tend to leak from polyurethanes, produce a fatty or oily surface, and thus shorten the lifetime of the PCM-equipped product. Moreover, the tack may be lost, an important property for certain applications. Exudation of entrapped materials from PU gels is to be expected since U.S. Pat. No. 4,404,296 (Schäpel) discloses the use of those gels as active-ingredient release compositions.
Another important group of commercially available solid-liquid phase change materials comprise fatty acids and fatty acid esters. The latter are available for phase change ranges between ca. −50° C. and about +100° C. and can be readily mixed in any ratio to target the desired phase change behaviour needed for an application. Nevertheless, fatty acids and fatty acid esters are not as compatible between each other as paraffins and tend to show miscibility gaps and a tendency to supercool. Thus, in most cases it is unforeseeable whether a fatty acid ester PCM will work within a specific polymer matrix or not, and whether the material will show substantial, small or no supercooling effects. Different matrix polymers may give rise to different supercooling behaviours for the same PCM. On the other hand, even homologous PCM can show very different behaviours in the same matrix polymer.
US 2011/0281485 A1 (Rolland et al.) discloses a composition comprising or produced from a blend of at least one fatty acid ester (PCM) and at least one ethylene copolymer, wherein the ethylene copolymer comprises a considerable amount of a polar co-monomer, preferably a vinyl acetate or an acrylic component. The focus lies on the development of a blend of a suitable copolymer composition with conventional, e.g. fatty acid phase change materials. The tendency to supercool could be reduced with increasing vinyl acetate content. Since the matrix polymer has to be adapted to the PCM, the material suffers from restrictions to tune other desired properties, e.g. mechanical properties.